Why multiple series resistor on single phase metering reference design with HLW8012 and BL0937 IC

Question

We need to implement energy monitoring in our smart plug. We are considering 2 ICs HLW8012 and BL0937 for this purpose and following are their reference designs in their data sheets.

I am trying to understand why multiple series resistors are used on VP pin in both reference designs (6 resistors of 470K in HLW8012 and same 6 resistors of 200K in BL0937). Taking an example of BL0937 I am wondering why 1.2MOhm is not used in case of BL0937. 6 series resistor of 200K also gives same resistance 1.2Ohm and we could select resistor power of 1.2MOhm same as any of 200K resistor wattage.

Does multiple series resistor on VP pin serves any other purpose except for ease of availability?

We are struggling with limited size and wand to reduce components to as minimal as possible. Our preference is to use single 1.2MOhm resistor instead of six 200K resistor.

Your help to understand the logic behind multiple series resistor on VP pin will be much appreciated.

Answer 1

The main reason is the voltage rating of the resistor. Finding a 1.2MOhm resistor in ,say, 0.25W with a voltage rating of 600V or so is more difficult or expensive than using a string of common garden variety resistors with maybe a 200V rating. As well, there is the issue of electromigration. The voltage gradient causes the atoms of the resistive element to migrate. This ends up causing the resistor to fail. Spreading the voltage gradient across a number of resistors solves the problem.

[edit] Since the application is connected to the mains supply, you need to have a higher voltage rating due to transients that are expected on the mains. Lightning, inductive loads being switched off etc generate voltage spikes well in excess of the 320V peak (for a 240VAC system) you'd normally expect. This is also why you need creepage distances on your pcb in excess of what you'd anticipate for just 240VAC.